The present invention relates to sampling and processing of analog signals, more specifically, to analog-to-digital conversion using optical techniques.
Analog electronic signals can be represented or reconstructed by a certain set of discrete sampled values or samples. Many electronic applications use samples of analog signals instead of the analog signals. For example, digital data processing and communication systems use digital data converted from samples of an analog signal to achieve improved noise immunity and processing flexibility in data processing and transmission. Conversion of analog signals to digital data can be accomplished by first sampling the analog signals into sampled values and then digitizing the sampled values in a desired form.
A sampling rate or sampling frequency describes the number of samples taken from an analog signal per unit time (e.g., one second). The minimum sampling rate should be equal to or greater than the Nyquist rate, which is double the highest frequency in an analog signal, in order to preserve the minimum information content in the original analog signal. Thus, a high sampling rate is desirable in converting an analog signal with signal components at high frequencies into digital form. In addition, an analog signal may be oversampled at a sampling rate much higher than the Nyquist rate to improve the signal-to-noise ratio and/or precision of a subsequent analog-to-digital conversion.
Sampling of analog electronic signals is usually accomplished electronically by using electronic circuits. Electronic sampling techniques and circuits are well developed. The maximum sampling rate achievable by an electronic circuit is generally limited by the response times of the electronic components and the circuit configuration. This further limits the conversion speeds of many electronic analog-to-digital converters.
Such speed limitation in electronic digital-to-analog conversion can be an obstacle to many applications that require high-speed analog-to-digital conversion. Real-time digital video in applications such as telecommunication and machine vision is one such example. The performance of the real-time digital video in the existing video delivery on the Internet and in video conferencing systems is not only limited by the bandwidth limitation in the data transmission channels but also limited by the analog-to-digital conversion rates.
Therefore, there exists a need for devices and techniques that provide high-speed sampling and analog-to-digital conversion.
The present disclosure includes devices and techniques for sampling analog signals at high sampling rates by using optical pulses in a special way. It further provides devices and techniques for converting such high-rate samples into digital data.
One aspect of the disclosure describes generation of a train of dense pulses comprising a sequence of pulses that have different pulse signatures so that one pulse is distinguishable from adjacent pulses and different pulses with a common pulse signature can be separated from other pulses to form a new pulse train. The pulses are xe2x80x9cdensexe2x80x9d in a sense that the pulse repetition rate is higher than the upper switching rate of many electronic devices.
One example of such dense pulse trains may include a sequence of pulses that are centered at different wavelengths. One device for generating this pulse train includes a mode-locked laser for producing optical pulses with a known pulse repetition period, a plurality of optical demultiplexer (e.g., xe2x80x9cdropxe2x80x9d filters) connected in series and each configured to separate adjacent oscillation modes in each pulse near a different center wavelength and to transmit remaining modes in each pulse such that each demultiplexer produces a train of pulses of the same pulse repetition period at a different center wavelength, and a plurality of optical multiplexers (e.g., xe2x80x9caddxe2x80x9d filters) connected in series to form an optical path and configured to respectively couple the plurality of pulse trains at different center wavelengths to the optical path with a delay relative to one another so as to form an interleaved dense pulse train. This interleaved dense pulse train has a shortened pulse repetition period and a sequence of pulses at different center wavelengths within one pulse repetition period of the mode-locked laser.
Another device for generating the above interleaved dense pulse train comprises a plurality of mode-locked lasers respectively producing optical pulses at different center wavelengths with the same pulse repetition period, and a plurality of optical multiplexers (e.g., xe2x80x9caddxe2x80x9d filters) connected in series to form an optical path and configured to respectively couple the plurality of pulse trains from the mode-locked lasers to the optical path to form the interleaved pulse train.
A second aspect of the disclosure includes sampling an analog signal by using the above train of dense pulses. An optical modulate is used to modulate a property of the pulses in the interleaved dense pulse train in response to an analog signal and therefore impose the information in the analog signal onto the optical pulses. The analog signal may be in various forms such as an electrical analog signal that drives the optical modulator or an optical analog signal that interacts with the optical pulses to produce the modulation (e.g., a wave-mixing device). The property of the pulses may be the amplitude, phase, polarization, or other parameters of the optical pulses. One simple implementation is amplitude modulation by using an optical amplitude modulator that is driven by an analog electrical signal. This converts the information in the analog signal into amplitude variation of the dense pulses at a sampling rate equal to the repetition rate of the dense pulses.
A third aspect of the disclosure involves separating the information-bearing dense pulse train into a plurality of pulse trains according to their pulse signatures. In the above wavelength interleaved pulse train, pulses at different center wavelengths are separated into different pulse trains each with the same center pulse wavelength. Each pulse train has less pulses per unit time than the original pulse train. Hence, the pulse repetition rate is reduced to the low pulse repetition rate of the original unmodulated pulse trains. One or more optical demultiplexers, such as a set of optical xe2x80x9cdropxe2x80x9d filters respectively corresponding to the different center wavelengths, may be connected in series in the optical path of the dense pulse train to perform the pulse separation and to generate a plurality of parallel signal channels at different center wavelengths and different time delays. Each signal channel may further include a photosensor for converting the modulated optical pulses into analog electrical pulses, and an electronic analog-to-digital converter for converting the analog pulses into digital data. The reduced pulse repetition rate of the pulse trains in the signal channels can be set to accommodate for the processing speed of the electronic analog-to-digital converters. The analog-to-digital conversion of all signal channels is performed electronically in parallel. The combined digital data from all signal channels includes all the information content of the analog signal if the repetition rate of the dense pulse train is equal to or greater than the Nyquist rate of the analog signal and hence can be used for subsequent digital signal processing.
One advantage of the systems in the disclosure is their unique optical sampling by dense pulses to achieve desired high sampling rates. A train of dense pulses comprising a sequence of a plurality of pulses at different center wavelengths can be formed to have a high pulse repetition frequency that is in general difficult, if not possible to achieve with electronic sampling devices. An increase in the sampling rate by a factor up to and or greater than 102 may be achieved.
Another advantage is an increased speed in analog-to-digital conversion. This is at least in part due to the optical processing in sampling and separating the modulated dense pulse train into multiple pulse trains at different center wavelengths and in part due to the parallel analog-to-digital conversion in the multiple signal channels.
These and other aspects and advantages will become more apparent in light of the accompanying drawings, the detailed description, and the claims.